IE67324B1 - Method for the removal of nutriments containing carbon nitrogen and phosphorus - Google Patents

Abstract

The process is of the type consisting in circulating the raw water to be treated in cells or biological filters containing immersed filter beds operating anaerobically and then aerobically or vice versa. According to the invention, a treatment module is employed comprising at least two cells or biological filters in series (for example an anaerobiotic biological filter and one to five aerobiotic biological filters) by alternating anaerobiotic and then aerobiotic phases cyclically, the entry of the stream to be treated always taking place into the anaerobiotic cell. Advantageously, each stage of the cycle is separated by a stage and calculated delay comprising a stoppage of the feeding of water to be treated.

Description

The present invention relates to the field of the treatment of water with a view to its purification by biological means, notably discharge water and waste water or any water containing phosphorus in various forms and, possibly, ammonia nitrogen™ It relates quite especially to a method designed to eliminate carbon, nitrogen and traces of phosphorus by biological dephosphatization on biofilters.

It is known that waste water can be treated by the biological method known as the activated sludge method wherein a biomass is kept in suspension in the presence of oxygen and carbon-containing nutriment. The bacteria present in the biomass degrade the polluted matter and are separated from the purified water in a clarifier, It is also possible to maintain autotrophic bacteria in the system, enabling the conversion of nitrogen ammonia into nitric nitrogen if a great sludge age of at least six days is adopted in the system. If a period of non-aerated (or anoxic) contact is incorporated into the system, it is possible to force the heterotrophic bacteria to degrade the nitrates in the presence of carbon-containing nutriment™ If, moreover, the bacteria are made to undergo a systematic alternation of the anaerobic and aerobic states, an excess accumulation of phosphorus compounds can be caused in the bacteria™ A method such as this, with the above-described principles, has been known since 1974 under the process name of BARDENPHO.

All the configurations currently available for the total elimination of nitrogen and phosphorus by biological means constitute variants of the abovementioned standard activated sludge method™ A variety of possible arrangements of the anaerobic, anoxic and aerobic zones has led to the development of several methods in the course of the past 20 years, notably those known by the names of Phostrip, A/O, Phoredox, UCT, Biodenipho, etc,.

These configurations share the common feature of having an anaerobic basin at the front of the hydraulic flow designed to expose the biomass to an alternation of aerobic/anaerobic conditions» To reduce the nitrates in the anaerobic sone, several internal circuits for the recirculation of the sludges between anoxic and anaerobic sones are used in order to obtain efficient performance in terms of biological dephosphatization.

In these methods, several factors restrict the efficiency of the dephosphatization these are notably the exposure time of the biomass to the anaerobic conditions which, most frequently, has to be limited to between 0,,5 and 2 hours, and the values of load per mass which remain low because of the constraints on the age of the sludges dictated by the nitrification» This restricts the application of the methods in question to the treatment of effluents, for which the COD (chemical oxygen demand)/total P (phosphorus) ratio is high, notably higher than 20. It is difficult, in these methods, to obtain very low residual rates of phosphorus owing to the impossibility of increasing the load of carbon-containing nutriment for heterotrophic bacteria and owing to the fact that the subsequent clarifier releases suspended solids that are highly rich in phosphorus. Thus, it is necessary to add an additional physical/chemical process to the treatment of waste water to achieve a very low residual rate of phosphorus.

More recently, other methods for the purification of waste water have been proposed in which it is no longer activated sludge systems with freely circulating bacteria that are used but so-called biofilter reactors are used wherein the bacteria are fixed to a support. Among techniques of this kind, we may cite the processes described in the published French Patents Nos. 2 604 990 and 2 632 947. These methods lead to excellent results in the elimination of carbon and nitrogen but, owing to the fixing of bacteria, they cannot provide for a satisfactory biological dephosphatization.

It has now been found that it is possible to resolve this problem of the high elimination of phosphorus in the operational context of biofilter ~ methods by a technique which can be used to obtain a maximum rate of carbon-containing substrate xn an anaerobic cell so as to achieve an absence of electron (O2 and N03) acceptors during the non-aerated phase and minimize the losses of suspended solids (bacteria) .. p According to the essential characteristic of the method of the invention in which, to eliminate the $ carbon-containing and phosphorus-containing nutriments by the circulation of the. waste water to be treated in biofilters with submerged filtering beds, successive phases of anaerobiosis and then aerobiosis are made to alternate cyclically, the entry of the flow to be treated taking place always in the filter under anaerobic conditions ..

If it is furthermore desired to eliminate the nitrogen, nitrifying bacteria are placed in the submerged filtering beds.

A method such as this has numerous advantages which will emerge throughout the following description. In particular, the state of fixation of the biomass makes the period of stay of the bacteria in the reactor independent of the hydraulic period of stay during the cyclical phases of the process. This enables the optimum exploitation of the capacities of storage and release of the phosphorus of the dephosphatizing colony. In addition, the carbon-containing substrate on the biomass under anaerobic conditions is always at an optimum rate owing to the volume of water that has undergone settling in the anaerobic cells. Furthermore, the simultaneous filtration in the biofilters in series makes it possible to obtain good SS (suspended solids) indices at the treated water outlet and to reduce the loss of phosphorus-rich biomass in the effluent. Furthermore, according to an improvement of the method, the washing of the filtering cells can be done at the end of the aeration cycles with a simply nitrified (nitrate-rich) wash water. This makes it possible to prevent the release of phosphorus by the washing sludges during a subsequent settling operation. It ill be noted, finally, that in the case of waste water to he treated where the COD/P ratio is very low, the process may be completed by a physical/chemical dephosphatissation through the simple injection of the dephosphatisation reagents at the outlet of the anaerobic phase where the effluent has a high concentration of P, from PO^, after the release.

According to a particularly advantageous embodiment of the method of the invention, the alternation of the anaerobic and aerobic phases is separated by a hold phase where the conveyance of the flow to be treated is interrupted.

In practice, for a cell in the anaerobic state, advantageously at least two aerobic cells are used. The average periods of treatment in the abovementioned cycle generally correspond to 1 h to 8 h for the anaerobiosis, 0.5 to 1 h for the hold phase and 4 to 24 h for the aerobiosis.

According to one embodiment of the invention, in which three to five aerated cells are used for example, the circulation of water and of oxygenated gas in these cells takes place in ascending co-currents, the oxygenated gas being sent substantially to the middle of each cell.

According to a variant of the method, the untreated water to be treated can be made to go, upstream of the system and according to the abovementioned cycle, into a sludge blanket reactor in the anoxic state. The untreated water is introduced in an upflow into this reactor.

According to another alternative embodiment, the flow of water to be treated can be made to go into a cell in the anoxic state positioned upstream from the treatment system according to the cycle of the method. For example, in this case, there will be the following elements in series: 1 anoxic cell, 1 anaerobic cell and 1 to 4 or more aerobic cells. The anoxic cell may also be placed downstream with respect to the set of anaerobic and aerated cells, provided that an external carbonated source is added.

The invention will be understood more clearly from the following more detailed description of embodimentsf such as the above-described ones, made with reference to the figures in the single appended drawings sheet wherein, in diagrammatic form: Figure 1 shows a filtering module for a set of filtering cells of a water purification station, comprising one anaerobic cell for three anoxic/aerobic cells? - Figure 2 shows another filtering module of the same type as that of Figure 1 but with the arrangement, at the beginning of the method, of an anoxic sludge blanket reactor? - Figure 3 shows another biofiltering assembly for a purification station in which, upstream from the set of anaerobic/aerobic cells, there is introduced an anoxic cell for the carrying out of the denitrification process which does not come under the abovementioned alternation cycle.

The symbols used to denote each filtering cell (anaerobic, anoxic, aerobic and hold phases) are given at the top right-hand corner of the appended drawing.

Furthermore, for each of the three exemplary embodiments, a full cycle of alternating treatment according to the invention is given in concordance with each Figure: 1 to 8, 1' to 8'? 1 to 8 with an indication, for each step, of the average time expressed in hours (h).

In the configuration shown in Figure 1, there is positioned, ahead of the treatment module, an anaerobic cell 10 and then, in series, cells 11, 12, 13 (their number may go up to five) that are anoxic/aerobic at the same time. The set of cells may be preceded by a settling tank 9. Each filtering cell is provided with a floating or non-floating submerged filtering bed, for example one with a material that is lighter for the water and has a granulometry of 2 to 6 mm. The aeration screen in the cells 11, 12, 13 is placed at the level 14 within the filtering bed so as to create an anoxic zone 15 within such as the above-described ones, made with reference to the figures in the single appended drawings sheet wherein: - Figure 1 shows a filtering module for a set of filtering cells of a water purification station, comprising one anaerobic cell for three anoxic/aerobic cells; - Figure 2 shows another filtering module of the same type as that of figure 1 but with the arrangement, at the starting or fore-part of the method, of an anoxic sludge bed reactor; - Figure 3 shows another biofiltering assembly for a purification station in which, upstream from the set of anaerobic/aerobic cells, there is introduced an anoxic cell for the carrying out of the denitrification process which does not come under the alternation cycle.

The symbols used to denote each (anaerobic, anoxic, aerobic and hold phases) are given at the too right-hand corner of the appended drawing.

Furthermore, for each of the three exemplary embodiments, a full cycle of alternating treatment according to the invention is given in concordance with each figures 1 to 3, 1' to Q; i" to 8" with an indication, for each step, of th® average time expressed in hours above-mentioned filtering cell MORE DETAILED DESCRIPTION In the configuration shown in figure 1, there is positioned, at the front or fore-part of the treatment module, an anaerobic cell 10 and then, in series, cells 11, 12, 13 (their number may go up to five) that are anoxic/aerobic at the same time. The set of cells may be preceded by a settling tank 9. Each filtering cell is provided, with a floating or non-floating filtering bed, for example one with a material that is lighter -for the water and has a granulometry of 2 to 6 aa. The aeration screen in the cells 11, 12,, 13 is placed at the level 14 within the filtering bed so as to create an anoxic zone 15 within each cell in an aerobic state. The water to be treated (arrow IS) reaches the anaerobic cell 10 in an upflow, then a flow of water and air is introduced in ascending co-currents during the aerobic phase as A part 17 of the throughput of recirculated and mixed with the indicated in figure 1. treated effluent 18 is flow coming out of the the variant indicated anaerobic call 10. According to by the dashes and the three arrows, a part 19 of the throughput of water that has undergone settling may be introduced directly into the anoxic sone 15 of the aerated cells 11, 12, 13 so as to promote the process of denitrification.

According to the configuration shown in figure 2, an anoxic upflow sludge blanket reactor 20 is positioned at the front of the treatment module so as to achieve the process of denitrification outside the filtration cells. The processes of oxidation of the organic phase, nitrification and biological dephosphatization, are carried out by the fixed biomass, as in the case of figure 1, on the carrier medium ©f the filtering cells. The sludge blanket in the reactor 20 is fed by all or a part of the untreated water 21 entering the station. The other part will be applied directly to the anaerobic pilot according to 21a (figure 2). According to one variant, a part 21a of the untreated water may be directed to the anaerobic filter 10. The reactor 20 also receives the recirculation throughput 22 of the effluent 23 from the aerobic cells. The reactor 20 thus plays the role of both a primary settling tank and a - 8 characteristics: total COD N (of NHJ Total phosphorus mg/1 less than 1 mg/1 less than 1 mg/1

Claims (10)

1. Method for the elimination of carbon and phosphorus from waste water by biological means, of the type consisting in circulating the water in brofilters with submerged filtering beds working anaerobically and then aerobically or vice versa characterised in that, in a system of at least two cells or biofilters in series, phases of anaerobiosis and then aerobiosis are alternated cyclically, the entry of the flow to be treated taking place always in the filter under anaerobic conditions,

2. Method according to Claim 1, for enabling furthermore, nitrogen to be eliminated, characterized in that nitrifying bacteria are placed in the submerged filtering beds.

3. Method according to Claim 1 or 2, characterized in that each of the phases of the treatment cycle is separated by a hold phase corresponding to an interruption of the supply of water to be treated.

4. . Method according to any one of Claims 1, 2 or 3 characterized in that, for a cell under anaerobic conditions, at least two cells under aerobic conditions are used, the average periods of treatment in the abovementioned cycle corresponding to: 1 h to 8 h for the anaerobiosis, 0.5 to 1 h for the hold phase and 4 to 24 h for the aerobiosis.

5. Method according to Claim 1, characterized in that three to five aerated cells are used per anaerobic cell.

6. Method according to any one of Claims 1 to 5 characterised in that, in the aerated cells, the circulation of water and of oxygenated gas takes place in ascending co-currents, the oxygenated gas or air being sent substantially to the middle of each cell, and the effluent being recirculated in a lower zone.

7. » Method according to any one of Claims 1 to 5, characterized in that upstream of the system and according to the abovementioned cycle, the untreated water to be treated is passed in an upflow into a sludge blanket reactor in the anoxic state supplied with the untreated water and a recirculation of the purified water.

8. Method according to any one of Claims 1 to 5 characterized in that, before the implementation of the 5 abovementioned cycle in a cell under anaerobic conditions and one to five cells under aerobic conditions, the flow to be treated is passed into a cell in the anoxic state positioned upstream of the said system supplied with the water to be treated and a recirculation of the purified 10 water.

9. Method according to any one of Claims 1 to 5 characterized in that, downstream with respect to the implementation of the abovementioned method in a cell under anaerobic conditions and one to five cells under 15 aerobic conditions, the effluent is passed into an anoxic cell.

10. A method for the elimination of carbon and phosphorus from waste water by biological means, substantially as hereinbefore described with reference to the accompanying drawings .